As far as I can determine, certain games don't play nice with Windows Explorer, especially in fullscreen mode. The end result is that Explorer freaks out and gets stuck in an unrecoverable state. It seems to behave weirder with non-standard resolutions and weaker GPUs, such as yours.
If Explorer ever crashes on you, either keep clicking on it until you get a prompt to close it (the "Poke-it-to-check-it's-dead" Approach) or Ctrl+Alt+Delete and then kill the Explorer process (the "I'm-not-taking-your-stuff" Approach). To restart explorer, open up the Task Manager and then either click the File>Run new task, enter "Explorer" without quotes and then press enter, or failing that, right-click on the "Task Manager" entry under Apps, select "Open File Location" and then double-click on Explorer.exe.
As for preventing this kind of situation, you may wish to try borderless windowed mode that is available in all Source games. To enable borderless windows, in Steam, right-click on the Source game of choice (in this case TF2), select Properties>Set Launch Options... and then enter the following:
-windowed -noborder
Select Ok>Close. From now on, your game will run in a fullscreened window without a border, which will allow you to seamlessly switch between the game and your desktop without Explorer dying off. This is also very handy for crash-prone games, like the Source games, as it makes shutting them down a cinch.
